As we move towards the realization of widespread fiber optic networks, it is becoming increasingly important to provide optical switching at the optical network nodes. Optical switching is expected to become increasingly important as wavelength division multiplexing expands the number of optical paths available. By using integrated optical components to perform network node routing functions, advantages in terms of functionality, size, speed, and efficiency are achievable.
The integrated optical multimode interference (MMI) coupler has been the subject of much attention and research in recent years, see for example: L. B. Soldano, et al. in a paper entitled "Planar Monomode Optical Couplers Based on Multimode Interference Effects," J Lightwave Technol., vol. 10, no. 12, pp. 1843-1849, 1992; M. Bachmann, et al. in a paper entitled "General self-imaging properties in N.times.N multimode interference couplers including phase relations," Appl. Opt., vol. 33, no. 18, pp. 3905-3911, 1994; and L. B. Soldano et al., in a paper entitled "Optical multi-mode interference devices based on self-imaging: principles and applications," J Lightwave Technol., vol. 13, no. 4, pp. 615-627, April 1995. All references in this document are herein incorporated by reference. This passive device has been shown to possess a host of desirable qualities such as low excess loss, small size, fabrication tolerant behavior, and relative polarization and wavelength insensitivity. It has also been shown that MMI couplers can be used in a generalized Mach-Zehnder interferometer (GMZI) configuration to actively route and switch optical signals, as detailed by: L. B. Soldano et al., in a paper entitled "Optical multi-mode interference devices based on self-imaging: principles and applications," J Lightwave Technol., vol. 13, no. 4, pp. 615-627, April 1995; and R. M. Jenkins, et al., in a paper entitled "Novel 1.times.N and N.times.N integrated optical switches using self-imaging multimode GaAs/AlGaAs waveguides," Appl. Phys. Lett., vol. 64, no. 6, pp. 684-686, February 1994.
An N.times.N GMZI has a limited switching capacity. The N.times.N GMZI has N possible switching states. In view of this, there are many desired switching states that are not accessible. Indeed, once a route has been chosen for light launched into a particular input port of the N.times.N GMZI to emerge from a selected output port, routes for light launched into all remaining input ports are fixed. For example, if light is switched from a first input port to a fourth output port in a 4.times.4 GMZI, light can only be switched from: a second input port to a second output port; a third input port to a third output port; and a fourth input port to a first output port. This demonstrates blocking switching capacity provided by an isolated N.times.N GMZI. 15 Switches have been proposed that use a plurality of Mach-Zehnder interferometers, see, for example, M. Bachmann, et al., "Compact Polarization-Insensitive Multi-Leg 1.times.4 Mach-Zehnder Switch in InGaAsP/InP," in Proc. ECIO, Firenze, Italy, pp. 519-522, 1994, in which a number of independently controlled 1.times.N GMZI switches are used. While this design is a strictly non-blocking optical switch, it requires 4N MMI couplers, 2N.sup.2 phase shifters, and numerous waveguide crossings, resulting in a large and complex switch with complicated control requirements. The waveguide crossings have specific geometrical tolerances that have to be met. If the specific geometrical tolerances are not met, "cross-talk" increases substantially and often increases attenuation. Control of a 4.times.4 switch using Bachmann's design requires 16 MMIs and 32 phase shifters. It will be appreciated by one skilled in the art that both manufacture and control of such a device is not a simple matter.
There is a need for reliable switches that are not overly complicated to manufacture and operate.